Generally speaking, presently existing display devices use a screen controller including a processor (which may be a dedicated processor) for generating signals to refresh a display of an image (which may be displayable in full or in part) by scanning along lines of successive character or point positions while simultaneously reading an image at each point or character position. The memory is read by means of an address word having one group of bits that is a function of the column number of the current point or character position along the current scan line, and another group of bits which is a function of the row number of the current line in a scan frame.
In most window display devices, the screen controller does not directly address the image memory per se when refreshing the display, but rather it addresses a "paste-up" memory in which another processor (or sven the screen controlling processor it self) has assembled the various "visible" portions of the image (i.e. the portions that are to appear in each window) by using fairly complex software to select the visible portions only from the total contents of the image memory per se.
The paste-up memory may store aspect words defining the image pattern at each point or character position. This is often true of display terminals for the image memory per se is located in a central computer which handles the contents of the paste-up memory by remote control, leaving the screen controller with the sole function of refreshing the display. This disposition has the drawback of reducing terminal independence and of monopolizing much of the memory capacity and processing time of the central computer.
One known way of giving more independence to a window display system, is to simplify the handling of the paste-up memory by splitting the screen and the displayable image into a relatively small number of unit blocks of equal size. The paste-up memory is then a small capacity memory limited to storing a single word for each unit block. This word defines the window which is to be visible in the corresponding block, and generally also defines which portion of that window is to be displayed. In either case, the screen controller uses these words to index its addressing of the image memory per se in such a manner as to read the appropriate aspect words for displaying the desired portion of the desired window. This disposition has the drawback of limiting the choice of windows in the displayable image to windows build up from unalterable unit blocks.
Preferred embodiments of the present invention remedy the above defects by means of a simply-constructed hard-wired circuit which continuously indicates the window to which the character or point position currently being refreshed belongs, and without imposing restrictions on window definition.